1. Field of the Invention
The present invention relates to a method for adjusting focus or tracking detection unit, and an optical disc device.
2. Background Information
Write-once and rewritable optical disc devices have become commonplace today. Microscopic tracks are provided in a spiral or concentric pattern on the optical discs used in these devices, and information is recorded on these tracks. To record information to the tracks or read information from the tracks, a light beam has to be controlled so that it is always located over the information tracks.
It is also necessary to correct deviation of the laser beam with respect to the recording surface due to axial runout of the optical disc, radial runout of the rotary axis of the turntable, or the like, so that the focal point of the laser beam precisely follows the recording surface of the optical disc.
FIG. 23 is a diagram illustrating the constitution of a conventional optical disc device. An optical irradiation unit 3 in FIG. 23 directs a light beam 2 at an optical disc 1 and irradiates the disc at a specific power. The irradiating light beam 2 passes through a beam splitter 5 and is focused by a focusing lens 4 on the information surface of the optical disc 1. The light beam 2 reflected by the optical disc 1 is directed by the beam splitter 5 to a light receiver 6. The light receiver 6 outputs the amount of received light as a signal. An optical head 7 is made up by the optical irradiation unit 3, the focusing lens 4, the beam splitter 5, and the light receiver 6. A focus error detector 10 detects a focus error signal (hereinafter referred to as FE signal) on the basis of the signal from the light receiver 6. The FE signal expresses runout between the focal position of the optical spot and the information signal recording surface of the optical disc. A total received light detector 12 detects the reflected light sum signal (hereinafter referred to as an AS signal) on the basis of the signal from the light receiver 6. A correction coefficient calculator 13 calculates a correction coefficient that is the ratio of the FE signal amplitude to the total amount of received light. When the total amount of received light changes, an automatic amplitude controller 14 (AGC circuit) automatically controls the amplitude of the FE signal based on the amount of change and the correction coefficient. More specifically, the automatic amplitude controller 14 outputs a value obtained by multiplying the basic gain by the quotient of dividing the FE signal by the AS signal. As a result, the FE signal amplitude is kept at a specific level even though the disc reflectivity may vary or there may be variance in the power of the light beam. This is because the FE signal and the AS signal are both generally proportional to the intensity of the light reflected from the disc.
A tracking error detector 11 detects a tracking error signal (hereinafter referred to as TE signal) on the basis of the signal from the light receiver 6. The TE signal is information about positional deviation in the track width direction of an optical pickup with respect to the pits. Just as with the FE signal, the total received light detector 12 detects the total amount of received light on the basis of the signal from the light receiver 6, and the correction coefficient calculator 13 calculates a correction coefficient that is the ratio of the TE signal amplitude to the total amount of received light. When the total amount of received light changes, the automatic amplitude controller 14 automatically controls the amplitude of the TE signal based on the amount of change and the correction coefficient. As a result, the FE signal amplitude is kept at a specific level even though there may be variance in the disc reflectivity or the power of the light beam. This is because the TE signal and the AS signal are both generally proportional to the intensity of the light reflected from the disc.
There has also been a proposal for a device that adjusts the amplitude of the FE signal or the TE signal when the amount of light reflected from an optical disc varies between tracks or between recording layers (see Japanese Laid-Open Patent Application 2002-170259, for example).
In addition, a device has been proposed in which attention is turned to the fact that changes in the FE signal are also caused by spherical aberration, and spherical aberration is imparted prior to focus pull-in, thereby increasing the slope of the S curve of the FE signal, allowing the amplitude thereof to be increased, and affording more reliable focusing (see Japanese Laid-Open Patent Application 2003-99970, for example).
With recently disclosed high-density optical disc devices that make use of blue lasers of about 405 nm, because of the short wavelength, considerable coma aberration occurs in the spot on the optical disc as a result of disc tilt. For example, compared to the red laser of a DVD, there is roughly 1.6 times as much coma aberration. Furthermore, when an objective lens with a large NA of 0.85 is used for narrowing the beam in addition to a blue laser, considerable spherical aberration occurs in the spot on the optical disc as a result of variance in the light transmitting layer thickness. For instance, compared to a lens of NA of 0.6, such as with a DVD, there is roughly 10 times as much spherical aberration.
Spherical aberration occurs when the actual light transmitting layer thickness of an optical disc deviates from the ideal light transmitting layer thickness that is used as a predetermined reference in the design of an optical head. As shown in FIGS. 20 and 21, when aberration occurs in the light spot on an optical disc, the detection sensitivity (that is, the amplitude or slope) of the FE signal or TE signal varies, but there is almost no change in the level of the AS signal. Therefore, when an automatic amplitude controller (AGC circuit) is constituted by a division circuit or the like as in the past, there is fluctuation in the level of the various outputs for focus and tracking AGC, and the gain of the focus control system and tracking control system fluctuates. In general, sensitivity and amplitude drop, so there is a decrease in gain, and in a worst case, focus control or tracking control deviate in a state in which the automatic amplitude controller (AGC circuit) has been actuated. Conversely, if the spherical aberration is corrected to be approximately zero after the adjustment of the loop gain of focus and tracking, the same problems as those encountered with the above-mentioned spherical aberration will occur with coma aberration depending on the configuration of the optical system or the direction, amount, and phase of the coma aberration that occurs.
Furthermore, although the focus or tracking error signal is adjusted or loop gain is adjusted under the condition that the spherical aberration is small, in the two-layer disc or multilayer disc, after the light beam moves between the layers, a big spherical aberration corresponding to difference of the light transmitting layer thickness occurs. Until the spherical aberration correction element sufficiently follows the spherical aberration, the gains of the focus and tracking lowers so that the focus or tracking control deviates on the information surface of the layer to which the light beam moved.
In light of the above situation, it is an object of the present invention to provide an optical disc device that allows automatic amplitude control capable of ensuring focus and tracking performance that will always remain stable even when spherical aberration or coma aberration occurs, and affords stable high functionality and reliability with both two-layer discs and multilayer discs.